EP3542185B1 - Scintillation detector for detecting and/or measuring radionuclides in a fluid - Google Patents

Description

TECHNICAL AREA

The field of the invention is that of the on-line detection and measurement of the radioactive contamination of a fluid. It relates more particularly to the detection and measurement of alpha and / or beta type radioactive contamination in a fluid pipe, in particular a liquid pipe, in particular for detecting and measuring online the presence of an alpha radioactive source. and / or beta in a drinking water pipe.

The fields of application of the invention are in particular radiation protection and safety.

STATE OF THE PRIOR ART

In order to avoid malicious, terrorist or simply accidental actions of radioactive contamination of fluid distribution networks, such as for example the air distribution networks of a clean room or the drinking water distribution networks, it is essential to be able to detect the presence of radioactive elements in the fluid as early as possible.

Currently, the continuous detection of gamma radiation in air or water is easily achievable and already has many solutions. The detection of beta or alpha radiation continuously (that is to say online) is much more difficult because of the short path in the air, and even more in water, of this radiation. .

By way of illustration, we have compiled in the table below the distances traveled by beta and alpha particles, respectively in air and in water, as a function of energy (data found in document [1 ] ). Energy Distance traveled by an alpha particle Distance traveled by an electron Air (12.04.10 ^-4 g.cm ^-3 ) Water (1 g.cm ^-3 ) Air (12.04.10 ^-4 g.cm ^-3 ) Water (1 g.cm ^-3 ) 5 keV 93.4 µm 989 Å 0.4 mm 450 nm 20 keV 313 µm 3775 Â 8 mm 8.5 µm 50 keV 660 µm 8150 Å 4cm 43 µm 100 keV 1.1 mm 1.3 µm 13.5cm 0.14 mm 500 keV 3.1 mm 3.6 µm 1.6 m 0.2 cm 1 MeV 5.2 mm 5.9 µm 4.1 m 0.4 cm 2 MeV 1 cm 11.4 µm 9 m 1 cm 3 MeV 1.7 cm 18.4 µm 13.7 m 1.5 cm 5 MeV 3.5 cm 37.3 µm 23 m 2.5 cm 7 MeV 5.9 cm 62 µm 31.2 m 3.5 cm 10 MeV 10.4 cm 110 µm 43 m 5 cm 15 MeV 20.5cm 215 µm 61.5 m 7.2 cm 20 MeV 33.6 cm 353 µm 78.4 m 9.3cm 50 MeV 1.7 m 1.8mm 161 m 19.8cm 100 MeV 5.83 m 6.3 mm 263 m 32.5cm

It can thus be seen that the path of a radiation, in water, ranges from 43 µm to 1 cm for beta rays with an energy ranging from 50 keV to 2 MeV and from 18 µm to 62 µm for alpha rays having an energy ranging from 3 MeV to 7 MeV.

Currently, the most widely used detection systems are semiconductor detectors or even scintillation detectors based on Inorganic scintillators or organic scintillators (in particular plastic scintillators) having relatively small detection surfaces, which requires an off-line analysis with measurement times greater than 1 hour. The classic method for performing this type of off-line analysis consists of taking a set of samples which are then sent to a laboratory where various examinations are carried out such as counting by liquid scintillation or a calorimetric method.

A system for detecting and measuring, online and continuously, radioactivity in a fluid is known from document [2] . This system allows the detection and measurement of beta, alpha, but also gamma contamination of a fluid, and in particular of water.

This system of the prior art is shown in the figure 1 : it has a detector 1 which is composed of two plastic scintillators in the form of a hollow cylinder (a cylinder 2 for gamma radiation and a cylinder 3 for beta and alpha radiation), nested one inside the other to form in the center a measuring chamber 4 in which the fluid to be controlled circulates. The fluid to be tested (for example, water) enters the measuring chamber 4 through a fluid inlet 5 located at one end of the measuring chamber and exits through a fluid outlet 6 located at the other end of the chamber. the measuring chamber. Each of the plastic scintillator cylinders is connected by a waveguide 7 to a photon detection device (not shown), for example a photomultiplier.

The efficiency of detection of beta and alpha particles of the system illustrated in the figure 1 is directly dependent on the ratio (V _m / V _t ), V _m being the measurement volume (i.e. the volume in which it is possible to detect beta and alpha particles) and V _t being the total volume of fluid contained in the measuring chamber. Since the distance traveled by the beta and alpha particles in a fluid is relatively small, the measurement volume V _m is small. The measurement efficiency of the detection system will therefore be lower the lower the detection limit.

In order to increase the occurrences of interactions between the alpha and beta particles and the plastic scintillator, it is proposed in document [2] to fill the measurement chamber of the system with beads made of a plastic scintillator material having a diameter between 250 µm and 500 µm. However, such a solution has the drawback of only taking into account the beads located directly facing the photomultiplier. In addition, due to the index difference between the fluid and the beads, there is poor propagation of the photons to the photomultiplier. All of these drawbacks result in a degradation of the detection efficiency (the measurement volume taken into account being smaller).

Furthermore, it is known from document [3] a detector of low energy radiation in a fluid, for example beta particles produced by the natural decrease of tritium in water. As shown in the figure 2 , this detector 10 comprises a cylindrical measuring chamber 11 having a fluid inlet 15 and a fluid outlet 16 and in which is disposed a bundle 13 of scintillating optical fibers 12, the proximal ends of which are connected to a photomultiplier 17, disposed outside of the measuring chamber 11. The detector described in document [3] makes it possible to increase the measurement volume (V _m ), the latter being directly linked to the number of fibers used. However, the arrival of the fluid 14 directly on the fibers generates turbulence. As the fibers are constantly in motion, the measurement geometry is constantly changing. These variations result in significant additional fluctuations in the count rate. These fluctuations can be considered as a multiplicative noise on the count rate and they can hardly be filtered out.

The inventors have set themselves the objective of improving the on-line controls of the contamination of a fluid by radionuclides and of improving the limits of detection of radionuclides in a fluid. In particular, they sought to design a system capable of measuring the alpha and / or beta environments of a fluid, continuously, without tapping or sampling.

DISCLOSURE OF THE INVENTION

• une chambre de mesure destinée à recevoir ledit fluide, ladite chambre comportant une entrée de fluide et une sortie de fluide afin d'autoriser une circulation du fluide dans la chambre ;
• un photomultiplicateur ;
• une pluralité de fibres optiques scintillantes regroupées ensemble pour former un faisceau de fibres, lesdites fibres optiques scintillantes étant optiquement connectée au photomultiplicateur, le faisceau de fibres étant au moins partiellement logé dans la chambre de mesure ;

le détecteur étant caractérisé en ce que la chambre de mesure est munie d'un premier et d'un deuxième séparateur délimitant une zone d'introduction comportant l'entrée de fluide, une zone d'extraction comportant la sortie de fluide et une zone de mesure, intermédiaire aux zones d'introduction et d'extraction, dans laquelle les fibres optiques scintillantes du faisceau sont déployées,
le premier et le deuxième séparateur étant chacun pourvu d'une pluralité d'ouvertures traversantes configurées pour établir un écoulement laminaire du fluide dans la zone de mesure.This objective is achieved by means of a scintillation detector for measuring and / or detecting radionuclides in a fluid, said detector comprising:

• a measuring chamber intended to receive said fluid, said chamber comprising a fluid inlet and a fluid outlet in order to allow circulation of the fluid in the chamber;
• a photomultiplier;
• a plurality of scintillating optical fibers grouped together to form a fiber bundle, said scintillating optical fibers being optically connected to the photomultiplier, the fiber bundle being at least partially housed in the measurement chamber;

the detector being characterized in that the measuring chamber is provided with a first and a second separator delimiting an introduction zone comprising the fluid inlet, an extraction zone comprising the fluid outlet and a measurement, intermediate to the introduction and extraction zones, in which the scintillating optical fibers of the bundle are deployed,
the first and the second separator each being provided with a plurality of through openings configured to establish a laminar flow of the fluid in the measurement zone.

In fact, by this particular configuration, one seeks to guarantee a laminar flow, homogeneous and non-turbulent for the fluid to be analyzed. It is recalled that a laminar flow of a fluid is a flow mode in which the whole of the fluid flows more or less in the same direction, without any local differences being opposed, as opposed to the turbulent regime , which is made of vortices that antagonize each other. Thanks to the detector according to the invention, the inventors have succeeded in improving the detection limits of radionuclides in a fluid by reducing the statistical fluctuations which appear during the presence of turbulence in the fluid circulating in the measurement chamber (we will also speak of fluid flow to designate this circulating fluid). Due to the particular design of the detector, the flow of fluid is regulated in the measurement zone, that is to say where the scintillating optical fibers are deployed. By minimizing the turbulence, the measurement geometry is more stable and the statistical fluctuations of the count rate are thus reduced and the detection limit is consequently improved.

Preferably, the through openings of the first and second separators are configured to evenly distribute the fluid in the measurement zone. This has the effect of regulating the flow of fluid in the measurement zone and reducing turbulence. This homogeneous distribution of the flow can be obtained by having a homogeneous distribution of the through openings of the first and second separators, as well as a homogeneous shape and size.

• la chambre de mesure comporte un corps latéral qui s'étend selon une direction longitudinale et chacun des premier et deuxième séparateurs comporte une plaque qui est disposée transversalement à la direction longitudinale et qui est solidaire du corps latéral, la pluralité d'ouvertures traversantes étant disposées dans la plaque ;
• le premier séparateur comporte en outre un orifice traversant qui est dimensionné pour permettre le passage du faisceau de fibres ;
• le premier séparateur comporte en outre un élément tubulaire qui est solidaire d'une face de la plaque, l'élément tubulaire et la plaque ayant un orifice traversant commun qui est dimensionné pour permettre le passage du faisceau de fibres, l'élément tubulaire étant configuré pour isoler le faisceau de fibres du fluide entrant dans la zone d'introduction ;
• le faisceau de fibres est agencé essentiellement parallèle à la direction longitudinale ;
• la plaque des premier et deuxième séparateur est un disque et l'orifice traversant du premier séparateur est un trou circulaire central ;
• les zones d'introduction, de mesure et d'extraction se succèdent selon la direction longitudinale et l'entrée de fluide et la sortie de fluide sont disposées transversalement à la direction longitudinale ;
• les ouvertures traversantes de la pluralité d'ouvertures traversantes du premier séparateur sont agencées selon un premier motif et sont équidistantes les unes des autres, et les ouvertures traversantes de la pluralité d'ouvertures traversantes du deuxième séparateur sont agencées selon un deuxième motif et sont équidistantes les unes des autres ; de préférence, les premier et deuxième motifs sont identiques ;
• la pluralité d'ouvertures traversantes du premier séparateur et la pluralité d'ouvertures traversantes du deuxième séparateur sont des trous circulaires ayant un même diamètre ;
• chacune des ouvertures traversantes du premier séparateur est disposée en face d'une des ouvertures traversantes du deuxième séparateur ;
• le faisceau de fibres comprend une portion proximale, qui est reliée au photomultiplicateur, et une portion distale, les fibres optiques scintillantes étant agencées serrées les unes contre les autres dans la portion proximale et espacées les unes des autres dans au moins une partie de la portion distale ;
• la portion distale du faisceau de fibres comprend au moins un élément configuré pour espacer les fibres les unes des autres. Il peut par exemple s'agir d'une résine, disposée entre les fibres, qui va permettre de les désolidariser et de les écarter les unes des autres.

Some preferred but non-limiting aspects of this detector are as follows:

• the measuring chamber comprises a lateral body which extends in a longitudinal direction and each of the first and second separators comprises a plate which is arranged transversely to the longitudinal direction and which is integral with the lateral body, the plurality of through openings being arranged in the plate;
• the first separator further comprises a through orifice which is dimensioned to allow the passage of the bundle of fibers;
• the first separator further comprises a tubular member which is integral with one face of the plate, the tubular member and the plate having a common through hole which is dimensioned to allow passage of the fiber bundle, the tubular member being configured to isolate the fiber bundle from fluid entering the introduction zone;
• the fiber bundle is arranged essentially parallel to the longitudinal direction;
• the plate of the first and second separator is a disc and the through hole of the first separator is a central circular hole;
• the introduction, measurement and extraction zones follow one another in the longitudinal direction and the fluid inlet and the fluid outlet are arranged transversely to the longitudinal direction;
• the through openings of the plurality of through openings of the first separator are arranged in a first pattern and are equidistant from each other, and the through openings of the plurality of through openings of the second separator are arranged in a second pattern and are equidistant from each other; preferably, the first and second units are identical;
• the plurality of through openings of the first separator and the plurality of through openings of the second separator are circular holes having the same diameter;
• each of the through openings of the first separator is arranged opposite one of the through openings of the second separator;
• the fiber bundle includes a proximal portion, which is connected to the photomultiplier, and a distal portion, the scintillating optical fibers being arranged tight to each other in the proximal portion and spaced apart from each other in at least a portion of the portion distal;
• the distal portion of the fiber bundle includes at least one member configured to space the fibers apart from each other. It may for example be a resin, placed between the fibers, which will make it possible to separate them and separate them from each other.

The invention also relates to a use of a scintillation detector as defined above for the detection of contamination by radionuclides in a flow of a fluid, preferably in a flow of drinking water.

The detector according to the invention has many advantages. In particular, it makes it possible to easily increase the measurement volume by increasing the detection surface, which is directly linked to the number and to the length of scintillating optical fibers used. In addition, the detector according to the invention makes it possible to detect beta and / or alpha radiation in a fluid, and in particular in water. For this, it will be preferable to use scintillating optical fibers with a single sheath and of small diameter (less than 300 μm), the thickness of the sheath being as thin as possible. Finally, the detector according to the invention has the advantage that its production is inexpensive.

• d'attaques terroristes visant à contaminer les réseaux de distribution d'eau potable (château d'eau, conduite d'alimentation, etc.) ;
• du contrôle de la contaminateur beta et alpha de l'eau, lors d'un accident avec risque de contamination.

The detector according to the invention has many applications. It can in particular be implemented for monitoring beta and alpha radioactivity in water distribution networks in the context of:

• terrorist attacks aimed at contaminating drinking water distribution networks (water tower, supply pipe, etc.);
• control of the beta and alpha contaminator of the water, during an accident with risk of contamination.

Other characteristics and advantages of the invention will appear more clearly on reading the additional description which follows and which refers to the appended figures.

Of course, this additional description is given only by way of illustration of the invention and in no way constitutes a limitation thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

• The figure 1 shows, in a sectional view, a detection system according to a first prior art, making it possible to detect ionizing radiation of gamma, alpha and beta type.
• The figure 2 shows, in a sectional view, a detector according to a second prior art.
• The figure 3 is a schematic representation, in a sectional view, of a first embodiment of the scintillation detector according to the invention.
• The figure 4 is a schematic representation, in a sectional view, of a second embodiment of the scintillation detector according to the invention.
• The figures 5a to 5d are schematic representations of the first separator according to a particular embodiment, respectively according to a perspective view ( figure 5a ), from below ( figure 5b ), On top ( figure 5c ) and in section ( figure 5d ) along line AA.

DETAILED PRESENTATION OF A PARTICULAR EMBODIMENT

The invention relates to a scintillation detector for measuring and / or detecting radionuclides in a fluid, comprising in particular a bundle of scintillating optical fibers, a photomultiplier and a first and a second separator for reducing, within a measurement area, turbulence around the scintillating optical fibers.

Scintillating optical fibers are well known to those skilled in the art. As they are optical fibers, they are very good optical guides and there is therefore no problem of collecting the photons induced by the scintillation. They make it possible both to produce the scintillation and to drive the photons induced by the scintillation towards the photomultiplier.

As a reminder, a typical scintillating optical fiber comprises a core made of a scintillator material (generally a solid plastic scintillator material), surrounded by one or more sheaths made of a material having a refractive index lower than that of the heart. The scintillator material is intended to convert ionizing radiation of alpha and / or beta type into photons. Preferably, the scintillating optical fibers are chosen with the smallest possible core diameter, in order to reduce the interactions of ionizing radiation of the gamma type and thus to be able to increase the measurement volume V _m by increasing the number of fibers.

The scintillating optical fibers most suited to the detector which is the subject of the invention are those having a single sheath, this sheath preferably having the thinnest possible thickness. In the case of a measurement in water, the refractive index of the water being lower than that of the core of the scintillating optical fiber, one could very well consider using the fiber without an outer sheath, which would have the advantage of increasing the detection efficiency, especially for alpha radiation.

The use of scintillating optical fibers is very advantageous. In fact, each scintillating optical fiber is seen as a detector offering a measurement volume linked to the surface thereof. It is therefore easy to increase the measurement volume by increasing the length of each fiber and / or by increasing the number of fibers in the bundle. You can also increase the diameter of the fibers, while keeping in mind that a small diameter fiber will be more flexible (therefore easier to deform to occupy a given space) and that it is preferable to have small diameter fibers. if one wishes to reduce the interactions of ionizing radiation of the gamma type. Preferably, scintillating optical fibers having a diameter of less than 300 μm are used.

It is preferable that the scintillating optical fibers are arranged, in the measurement zone, so as to maximize a contact surface between the scintillating optical fibers and the fluid to be analyzed, in order to maximize the measurement volume V _m and obtain a ratio. V _m / V _t close to 1. To do this, it is advantageous to use, in the distal part of the bundle intended to be placed in the measurement zone, a means for keeping the fibers apart. For example, a resin can be placed between the fibers. This is also the reason why, according to a preferred configuration of the invention, the lateral body of the measuring chamber extends in a longitudinal direction and the scintillating optical fibers of the fiber bundle extend essentially parallel to this direction. longitudinal.

It is also recalled that, if it is desired to measure the ionizing radiation of alpha and / or beta type, it is preferable that the scintillating optical fibers do not touch each other in the measurement zone. In fact, if the fibers of the bundle touch each other throughout their length, this amounts to having a single fiber having the diameter equivalent to that which the bundle has in its proximal portion. However, the larger the diameter of a scintillating optical fiber, the more it will tend to be sensitive to gamma radiation, and this radiation having an intensity greater than alpha and beta radiation, the latter will be masked by gamma radiation.

The photomultiplier as a photon detection device is well known to those skilled in the art. It converts the photons induced by the scintillation into corresponding electrical signals. Specifically, the photomultiplier converts photons into electrons and multiplies them in order to build an electrical impulse. It may for example be an H10720 type photomultiplier from Hamamatsu. The photomultiplier can be associated with a filtering module allowing the denoising of the electronic pulses (which makes it possible to discriminate the signals that are too weak) and with a counting and smoothing module giving a count rate value (which makes it possible to measure the number of beta and / or alpha particles present in the fluid). The filtering module and the counting module are functions integrated into an electronic processing board. These 2 functions (filtering and counting) are classic functions found on any acquisition card of this type.

One represented on the figure 3 a diagram of a detector 10 according to a first embodiment of the invention.

The detector 20 comprises a measuring chamber 21 equipped with a fluid inlet 25 and a fluid outlet 26, a photomultiplier 27 and a bundle 23 of scintillating optical fibers 22, the bundle comprising a proximal portion 28, connected to the photomultiplier 27, and a distal portion 29 intended to be deployed in the measurement zone 41.

The measuring chamber 21 comprises a lateral body 36 which extends in a longitudinal direction between two ends 37, 38 (it is preferably a cylinder) and the bundle 23 is housed in the measuring chamber, the bundle having its fibers arranged substantially parallel to the longitudinal direction. The photomultiplier 27 is located at one of the ends 37 of the side body.

In the figure 3 , the fluid inlet 25 and outlet 26 are located close to the two ends of the lateral body of the measuring chamber, in order to maximize the dimensions of the measuring zone 41. They are also arranged transversely with respect to the direction longitudinal measurement chamber 21 (they are furthermore placed on the same side of the lateral body, but they could also have been located on two opposite sides).

As a person skilled in the art knows, the side body 36 must be made of a material opaque to UV radiation or / and placed in the dark, so as not to generate scintillations due to ambient light and, above all, in order not to not saturate the photomultiplier.

In the context of the invention, the volume of the measuring chamber 21 is divided into several zones using a first 30 and a second 31 separator. The first separator 30 makes it possible to separate the introduction zone 40 (comprising the fluid inlet 25) and the measurement zone 41, while the second separator 31 makes it possible to separate the measurement zone 41 from the extraction zone 42 (including the fluid outlet 26).

In the embodiment illustrated in figure 3 , the measuring chamber 21 being of generally cylindrical shape and of circular section, the first 30 and second 31 separators are plates 32, which have the form of discs, pierced with through openings 33 such as perforations, in this case circular holes.

Furthermore, since the photomultiplier 27 is located at one end 37 of the measuring chamber and the beam 23 is arranged parallel to the longitudinal direction of the chamber, the first separator 30 further comprises an orifice 34 dimensioned to allow insertion. of the beam 23. Preferably, this orifice is central and is circular.

According to a preferred embodiment illustrated in figure 4 , the first separator 30 further comprises a tubular element 35, one end of which is integral with the disk-shaped plate 32 and the other end of which is pressed against one of the end walls 37 of the measuring chamber. Thus, the tubular element 35 isolates the beam 23 from the flow entering the introduction zone 40 and the beam opens directly into the measurement zone 41 where the turbulence is less.

In the figure 4 , the second separator 31 is identical to the first separator 30 and it also has a tubular element 35. To prevent the orifice 34 of the tubular element from creating turbulence, the end of the tubular element of the second separator is pressed against against the other of the end walls 38 of the measuring chamber. It should be noted that we could also have used, for the second separator 31, a plate 32 in the form of a disc and drilled like that used in the figure 3 .

For the through openings 33 of the first and second separators, it is preferable that, within the same separator, they are identical (same shape and same size) and arranged in a homogeneous manner. It is also preferable that they are identical and arranged in the same way on the two separators.

For each separator, the number of through openings 33, their shape, size and arrangement are chosen so as to establish a flow laminar of the fluid to be analyzed in the measurement zone. Of course, the size of the through openings will be chosen smaller than the size of the fluid inlet 25 and outlet 26.

In the figures 5a to 5d is shown a possible configuration of the first and second separators used in the figure 4 . The disc-shaped plate 32 has a diameter of 59.5 mm, a thickness of 5 mm, and a central hole 34 of a diameter of 20.5 mm; the tubular member 35 is a straight tube having an internal diameter of 20.5 mm and an external diameter of 28 mm and a height of 23.5 mm. The disc is provided with 30 circular holes, arranged in 3 circles of 10 holes centered on the same central axis as that of the central hole; the holes have a diameter of 5 mm; the inner circle 43, the intermediate circle 44 and the outer circle 45 respectively have a diameter of 35 mm, 43 mm and 50 mm; the holes of each of the circles are arranged at an angle of 36 ° to the adjacent holes located on the same circle.

The detector 20 according to the invention can be placed in a water distribution circuit, for example by being placed between two pipes or even in a configuration similar to that used for the installation of the filter cartridges. This thus allows all the water flow circulating in the distribution circuit to pass through the scintillation detector, without having to make a bypass.

To illustrate the invention, we have produced a detector as illustrated in figure 4 , the first and second separators having the configuration illustrated in the figures 5a to 5d . The measuring chamber 21 is a cylinder having an internal diameter of 80 mm and an internal height of 180 mm, ie an internal volume of 0.90 liters. The fluid inlet and outlet are 1.27 cm (½ inch) in diameter.

• matériau du cœur: polystyrène ayant une densité de 1,05 ;
• indice de réfraction du cœur: 1,60 ;
• matériau de la gaine : poly(méthacrylate de méthyle) (PMMA)
• indice de réfraction de la gaine : 1,49
• pic d'émission : 432 nm ;
• temps de décroissance : 2,7 ns ;
• 1/e (pour une fibre ayant un diamètre de 1 mm) : 2,2 m ;
• nombre de photons par MeV : environ 8000

In our exemplary embodiment, we use scintillating optical fibers bearing the reference BCF-10 from Saint-Gobain and which have the following characteristics:

• core material: polystyrene having a density of 1.05;
• refractive index of the heart: 1.60;
• sheath material: poly (methyl methacrylate) (PMMA)
• refractive index of the cladding: 1.49
• emission peak: 432 nm;
• decay time: 2.7 ns;
• 1 / e (for a fiber having a diameter of 1 mm): 2.2 m;
• number of photons per MeV: about 8000

The BCF-10 reference fibers are available with a circular or square cross section, with a diameter or a side ranging from 0.25 mm to 5 mm. The thickness of the sheath represents 3% of the diameter of the fiber for fibers with a round section.

où L est la longueur des fibres scintillantes, N est le nombre de fibres scintillantes, r est la distance parcourue par les particules considérées dans le fluide et d est le diamètre des fibres scintillantes.The measurement volume V _m is directly related to the number and to the length of the fibers used (having a circular cross section) according to the following formula: $${\mathrm{V}}_{\mathrm{m}}=\mathrm{\pi }\times \mathrm{L}\times \mathrm{NOT}\times \mathrm{r}\times \left(\mathrm{r}+\mathrm{d}\right)$$

where L is the length of the scintillating fibers, N is the number of scintillating fibers, r is the distance traveled by the particles considered in the fluid and d is the diameter of the scintillating fibers.

We chose to use 1000 single-sheathed BCF-10 fibers having a core diameter of 0.25 mm and a length of 30 cm and they were bundled together to form a bundle.

The fluid which is introduced into the measuring chamber is here water.

If we consider beta particles of 500 keV energy, we have a distance r = 0.2 cm, as indicated in the table above and the detection volume is 4.24 liters.

For beta particles of 100 keV energy, we have a distance r = 0.14 mm and the detection volume is 0.051 liter.

Consequently, by using a bundle of 1000 fibers as defined above for a volume of water to be analyzed, which is for example of the order of 0.15 liters, the detection efficiency of beta particles of 500 keV is 100% and 34% for those of 100 keV.

The detector according to the invention allows use with various fluid volumes beyond 0.15 liters, while retaining the measuring efficiency of the detector. In our exemplary embodiment, the detector produced having a measurement chamber volume of the order of 0.9 liters, if it is desired to obtain 100% efficiency of detection of beta particles of 500 keV and 34% for those of 100 keV, it will be necessary to proportionally increase the number of optical fibers, ie use 5829 fibers. For information, by using a bundle of 1000 fibers in a chamber volume of 0.9 liters, we obtained a beta particle detection efficiency of 100 keV equal to 5.8%.

CITED REFERENCES

1. [1] table found on the website http://www.cloudylabs.fr/wp/portee-des-particules
2. [2] US 2008/0260100 A1
3. [3] US 5,793,046

Claims (13)

1. Scintillation detector (20) for measuring and/or detecting radionuclides in a fluid, said detector comprising:

   - a measuring chamber (21) intended to receive said fluid, said chamber comprising a fluid inlet (25) and a fluid outlet (26) in order to authorise a circulation of fluid in the chamber;

   - a photomultiplier (27);

   - a plurality of scintillating optical fibres (22) grouped together to form a bundle (23) of fibres, said scintillating optical fibres being optically connected to the photomultiplier, the bundle of fibres being at least partially housed in the measuring chamber;

   the detector being characterised in that the measuring chamber (21) is provided with a first (30) and with a second (31) separator delimiting an introduction zone (40) comprising the fluid inlet (25), an extraction zone (42) comprising the fluid outlet (26) and a measuring zone (41), between the introduction and extraction zones, wherein the scintillating optical fibres of the bundle are deployed,
   the first and the second separator each being provided with a plurality of through-openings (33) configured to establish a laminar flow of the fluid in the measuring zone (41).
2. Detector according to claim 1, wherein the measuring chamber (21) comprises a lateral body (36) that extends along a longitudinal direction and each one of the first and second separators comprises a plate (32) which is arranged transversally to the longitudinal direction and which is integral with the lateral body, the plurality of through-openings (33) being arranged in the plate.
3. Detector according to claim 2, wherein the first separator (30) further comprises a through-orifice (34) that is sized to allow for the passage of the bundle of fibres.
4. Detector according to claim 2, wherein the first separator (30) further comprises a tubular element (35) that is integral with a face of the plate (32), the tubular element and the plate having a common through-orifice (34) that is sized to allow for the passage of the bundle of fibres, the tubular element being configured to isolate the bundle of fibres from the fluid entering into the introduction zone.
5. Detector according to claim 3 or claim 4, wherein the bundle of fibres is arranged substantially parallel to the longitudinal direction.
6. Detector according to claim 5, wherein the plate (32) of the first and second separator is a disc and the through-orifice (34) of the first separator is a central circular hole.
7. Detector according to any of claims 2 to 6, wherein the introduction, measuring and extraction zones follow one another along the longitudinal direction and the fluid inlet and the fluid outlet are arranged transversally to the longitudinal direction.
8. Detector according to claim 1 to 7, wherein the plurality of through-openings of the first separator are arranged according to a first pattern and are equidistant from one another and the plurality of through-openings of the second separator are arranged according to a second pattern and are equidistant from one another, the first and second patterns being preferably identical.
9. Detector according to claim 1 to 8, wherein the plurality of through-openings of the first separator and the plurality of through-openings of the second separator are circular holes having the same diameter.
10. Detector according to claim 1 to 9, wherein each one of the through-openings of the first separator is arranged facing one of the through-openings of the second separator.
11. Detector according to claim 1 to 10, wherein the bundle of fibres comprises a proximal portion (28), which is connected to the photomultiplier, and a distal portion (29), the scintillating optical fibres being arranged tightly against one another in the proximal portion and spaced from one another in at least one portion of the distal portion.
12. Detector according to claim 11, wherein the distal portion (29) of the bundle of fibres comprises at least one element configured to space the fibres from one another.
13. Use of a scintillation detector such as defined in any of claims 1 to 12 for detecting a contamination by the radionuclides in a flow of a fluid, preferably in a flow of drinking water.

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